sedimentary
Chert (Flint Nodular Variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Gray, tan, brown with banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Gray, tan, brown with banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Gray, tan, brown with banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the replacement of limestone or chalk by silica-rich fluids. Often found as nodules or layers within sedimentary rock sequences, originally accumulating as diatomaceous or radiolarian ooze on the ocean floor.
Uses & applications
Historically used for stone tools (knapping) and sparking fire. In modern times, used as an aggregate in construction, road metal, and occasionally for lapidary work/jewelry when highly colored (jasper).
Geological facts
Chert is so durable that it often persists long after the surrounding limestone has weathered away. It was one of the most important resources for prehistoric humans to create sharp-edged tools like arrowheads and knives.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture pattern and its ability to scratch glass. It often has a thin white outer 'rind' (cortex) if found within chalk. Commonly found in riverbeds or coastal cliffs.
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Sandstone or Siltstone
Arenite (if sandstone)
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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